This invention relates to the field of pulse-width modulation systems for causing currents to flow through a load in response to a command signal. The invention is particularly useful in the field of motion control, such as in servo-amplifiers, brushless motors, and the like. It is also especially suitable for use in driving the gradient coils in a magnetic resonance imaging (MRI) system.
It has been known to control a motor with a servomechanism, wherein the current through the motor is held at a desired value, the desired value being represented by a command signal. The servomechanism regulates the current in the motor by comparing the command signal voltage with a feedback voltage, the latter being an appropriately scaled signal representative of the motor current. The difference between the command signal and the feedback signal is called the "error signal", and is used to drive an amplifier which applies current to the load.
Linear power amplifiers have been used for increasing the level of the error signal, so as to provide a signal capable of driving the motor. However, linear amplifiers dissipate power, and this power dissipation substantially reduces the efficiency of the system. A system which dissipates power must be provided with heat sinks, cooling fans, and similar apparatus, and the system's size and weight is therefore increased. The energy used to develop the power dissipated in the amplifier is wasted, increasing the overall cost of operation. Furthermore, excessive heat is known to shorten the useful lives of the semiconductor devices used in the amplifier.
Because of the above-described disadvantages of the linear amplifiers used to drive motors, it has been recognized that it is preferable to provide a power stage which does not dissipate power. A pulse-width modulated (PWM) circuit approaches this goal. In a pulse-width modulated circuit, the command signal is used to generate a train of pulses, the width of each pulse being related to the instantaneous value of the command signal. The pulses are generated by using a comparator to compare the command signal with a dither signal, which is a sawtooth or triangular wave. When the command signal exceeds the dither signal, the output of the comparator is high; at other times, the output of the comparator is low. The comparator output thus comprises the train of pulses representing the command signal.
The pulses are then used to drive an electronic switching device, such as one or more transistors, for intermittently applying a voltage across the load. When transistors are used as switches, they are either fully on (i.e. saturated) or fully off ("cut-off"). Thus, virtually no power is dissipated in the transistors, because when the transistors are saturated, there is almost no voltage drop, and when they are cut-off, there is negligible current flow. Thus, in effect, a PWM circuit comprises a switch for applying the voltage of the power supply across the load, wherein the switch does not itself consume appreciable power.
In practice, transistor switches do consume small amounts of power, because they are never totally cut-off or resistance-free. But the efficiency of a PWM circuit can be as high as about 90-95%, compared with only about 40% for linear amplifiers.
One problem associated with PWM circuits is current ripple. When a voltage is suddenly applied across an inductive and resistive load, such as an electric motor, the current through the motor rises almost linearly with time. When the voltage is then turned off, i.e. at the trailing edge of a voltage pulse, the current through the motor does not immediately fall to zero, but decreases approximately linearly with time, as the inductor's magnetic field collapses. Thus, the input voltage pulses applied across the load result in a current which has a ripple. This ripple is inherent to all PWM amplifiers. To some extent, the inductance of the motor acts as a filter for smoothing the ripple. The magnitude of the ripple is directly proportional to the supply voltage and inversely proportional to the switching frequency and the inductance of the motor. However, in most applications, the inductance of the motor and the power supply voltage are fixed. The easiest way of reducing the ripple is therefore to increase the switching frequency. However, doing so increases switching losses in the transistor switches.
The amount of ripple is further increased by the fact that, in PWM circuits of the prior art, the maximum voltage excursion is twice the magnitude of the supply voltage. Thus, if the supply voltage is designated as V, the switching network which produces the pulses alternately applies voltages of +V and -V across the load. Thus, the maximum excursion is 2V. The greater the voltage excursion, the greater the current ripple.
Current ripple is undesirable because it wastes energy in the motor. The ripple current waveform has both an average value and an rms value. The motor can respond only to the average current. If the current has any ripple, the rms value is larger than the average value. The difference between the rms current and the average current contributes only to wasteful heating of the motor, thus reducing efficiency. In order to achieve maximum efficiency, the average and rms currents must be equal. This condition occurs only when there is no ripple, i.e. when there is a constant DC waveform.
The present invention provides a circuit which, for a given basic frequency of the dither signal, and a given magnitude of the supply voltage, produces a PWM signal which reduces current ripple in the load by a factor of four, as compared with PWM circuits of the prior art. The invention also reduces the ripple current flowing through the filter capacitor. The invention comprises a unique bridge circuit which defines electrical paths for applying a voltage of the desired polarity across the load, at any given instant.